The advent of DWDM fibre optics telecommunications systems in the early 1990s have enabled a dramatic increase in the transmission capacity over point-to-point communication links. This was achieved through multiplexing of a large number of individually modulated light beams of different wavelengths onto the same optical fibre. Typical systems installed today would have 64 or more independent channels precisely aligned onto an ITU-T standardized grid at 100 GHz, 50 GHz or even narrower channel spacing. With modulation speeds of routinely 10 Gb/s and attaining 40 Gb/s in laboratory experiments, it is not unusual to obtain aggregated capacities in the order of several terabits per second of information being transmitted onto a single optical fibre (S. Bigo, Optical Fibre Communication conference, WX 3, Anaheim, 2002). At the same time, electrical switching capacities have been growing at a much slower rate, with current largest electrical matrices limited to typically 640 Gb/s in a single stage. Furthermore, in most of the switching nodes, a large fraction—typically 70%—of the traffic is distant traffic that just travels through the node. It is therefore advantageous to have optical devices with large pass-through capacity and local tuneable drop capability. This device is referred to in the literature as a Reconfigurable Optical Add-Drop Module or ROADM (J. Lacey, Optical Fiber Communication conference, WT, Anaheim, 2002).
A ROADM usually includes an input port for receiving a DWDM signal, an output port for the express traffic and at least one add or drop port(s) for adding or dropping wavelength channels for local processing. This is usually realized through the subsequent steps of demultiplexing the incoming DWDM input, providing an array of switching means to route the individual channels to either the output express port or the add/drop port, and multiplexing the express channels onto a single output port. Some ROADM have multiplexed add/drop ports, some provide fully demultiplexed add/drop ports.
It is known to one skilled in the art that multiplexing/demultiplexing technologies can be done in many different ways. Serial filter embodiments (Fibre Bragg Grating, Thin Film Filters, fibre Mach Zehnder cascade, birefringent filters, etc.) are usually limited in number of wavelength channels due to insertion loss impairments. Therefore, the two solutions of choice currently being developed for ROADM applications with a large number of wavelength channels are based on parallel wavelength filtering: either free-space embodiments using bulk diffraction gratings or waveguide embodiments using AWG (Arrayed Waveguide Gratings).
Free-space optics implementations usually comprise optical fibre ports, lens elements, one bulk diffraction grating and an array of switching means. For example, Corning Inc. from Corning, N.Y., supplies such a device based on a liquid-crystal switching element. Although showing superior optical performances, free-space optics solutions are typically expensive, due to extremely tight alignment tolerances of multiple high precision optical elements. Furthermore, the relative positioning of these elements must be maintained over a wide range of environmental conditions requiring elaborate opto-mechanical designs.
Paper PD FB 7 presented at OFC'02 in March 2002 in Anaheim, Calif. provides a wavelength selective switch such as shown by way of example in FIG. 11. The switch includes input coupling optics 1200, switching elements 1202, a main lens 1204, and a single diffraction grating 1206. Disadvantageously, in this embodiment, only a small part of the service of the diffraction grating 1206 lies in the focal plane of the main lens 1204. This prevents light beams from all ports to stay in focus. Integrated optics solutions on the other hand have the potential to maintain the relative positioning of the different elements put onto the same substrate. There are two main ways of performing parallel wavelength demultiplexing in waveguides: either using AWG or using Echelle grating, the former being by far the more popular device due to the difficulty of manufacturing high precision diffraction gratings in waveguide substrates. Bragg gratings have also been employed for this purpose.
The AWG was invented by Dragone (C. Dragone, IEEE Photonics Technology Letters, Vol. 3, No. 9, pp. 812–815, September 1991) by combining a dispersive array of waveguides (M. K. Smit, Electronics Letters, Vol. 24, pp. 385–386, 1988) with input and output “star couplers” (C. Dragone, IEEE Photonics Technology Letters, Vol. 1, No. 8, pp. 241–243, August 1989). The AWG can work both as a DWDM demultiplexer and as a DWDM multiplexer, as taught by Dragone in U.S. Pat. No. 5,002,350 (March 1991).
An integrated optics ROADM would therefore consist of an input AWG to demultiplex the input DWDM stream, an array of switching means to route the demultiplexed channels to either an express path or the drop ports, and an output AWG to multiplex the output express DWDM stream. Due to the cyclic nature of the AWG's filtering function, it is possible to use only one AWG to perform the ROADM function with some loop back (O. Ishida et al., IEEE Photonics Technology Letters, Vol. 6, No. 10, pp. 1219–1221, October 1994). Typically, interconnects in an integrated optics ROADM are done primarily using guided way optics, for example using waveguides.
The switching elements can either be integrated onto the same substrate as the AWG or can be hybridized. All-integrated embodiments typically make use of thermo-optical switches (see for example C. R. Doerr et al., IEEE Photonics Technology Letters Volume 15, No. 1, January 2003, p 138 to 140), taking up a lot of substrate area and requiring careful heat management, eventually limiting its scalability. Integrated MEMS-waveguide solutions have also been proposed in the past, but the switching element is usually limited to 1×2 or 2×2, therefore also limiting scalability (M. Katayama et al., Optical Fibre Communication conference, WX4-1, Anaheim, 2001). It is known to a man skilled in the art that hybrid embodiments are possible in which AWG output waveguides are coupled to MEMS switching elements through a micro-lens array. However, this usually leads to poor spectral performance, i.e. no wide flat channel shape passband (R. Ryf et al., European Conference on Optical Communications, PD B.1.5, Amsterdam, 2001).